This invention relates to the anodic polishing of surfaces of intermetallic niobium compounds and niobium alloys, in general and more particularly to an improved method for such polishing.
For various applications, particularly a-c applications, of superconductors which either consist entirely or at least have a surface layer of an intermetallic niobium compound, e.g., Nb.sub.3 Sn, or of a niobium alloy such as, for instance, niobium with 25 atom -% zirconium or niobium-titanium alloys with about 25 to 60% by weight titanium, it is of interest to have surfaces which are as smooth as possible and which are free of impurities and disturbances. In superconducting cavity resonators of niobium which are provided with an Nb.sub.3 Sn layer at their inside surface which is exposed to a high frequency field, for instance, a smooth surface, free of disturbances, for this Nb.sub.3 Sn layer is a condition for attaining a high critical flux density and a high Q factor. For, the alternating electromagnetic high frequency fields penetrate only some 10.sup.-8 m deep into the superconductor surface. The same is true for the depth of penetration of a-c currents in other a-c applications of Nb.sub.3 Sn. An example of such an application is use in superconductor cables, where Nb.sub.3 Sn can have advantages because of its critical current density which is substantially higher than that of niobium. Because of this small depth of penetration, the physical condition of the Nb.sub.3 Sn surface is of decisive importance for its application as an a-c superconductor. In particular, a surface which is disturbed, for instance, by roughness or impurities, can lead to an increase in the surface resistance and thus to increased a-c losses in the surface. High losses, however, result in an undesirable development of heat and, in the case of superconducting resonators, in particular, in a decrease of the Q factor and a reduction of the cirtical flux density.
Mechanical polishing methods are not well suited for achieving the desired smooth and disturbance-free surfaces. The same is true of chemical polishing methods, which lead to etching of the surface.
A method suited for treating superconductive Nb.sub.3 Sn surfaces, particularly for removing impurities from such surfaces, is, in fact, the method described in the journal "IEEE Transactions on Magnetics", vol. MAG-11, No. 2, March 1975, pages 420-422. In the disclosed method an oxide layer is generated on the Nb.sub.3 Sn surface by anodic oxidation of an aqueous ammonia solution. The layer is subsequently dissolved again chemically by means of hydrofluoric acid. However, since in this method the oxide layer formed is only fractions of a micrometer thick, a surface layer also only a fraction of a micrometer thick is removed when the oxide layer is dissolved. To remove surface layers 1 micrometer or more thick, the anodic oxidizing and the subsequent dissolving of the oxide layer would therefore have to be repeated many times. This is very expensive and time consuming, in particular because of the continuous changing between the ammonia solution and the hydrofluoric acid which is required. In addition, peaks or corners protruding from the surface can also be removed only by oxidizing and chemically dissolving the oxide layers many times. Because of the field concentration at such a peak, it is preferred that the oxide layer be formed there first. However, as soon as the latter has reached a given thickness, the growth of the thickness at the peak comes to a standstill until the oxide layer on the rest of the surface has reached a corresponding thickness. The surface remaining after the oxide layer is dissolved is therefore still a relatively accurate facsimile of the original surface, at least after only a few repetitions of oxidizing and dissolving.